A selective catalytic reduction (SCR) device is typically used as part of a vehicle exhaust system to reduce NOx gasses before the exhaust is discharged into the atmosphere. Diesel engines and, to a lesser extent, other internal combustion engines generate nitrogen oxide (NOx) gasses as byproducts of the fuel combustion process.
NOx gasses may be present in an exhaust stream in various forms, including as nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). Selective catalytic reduction (SCR) is a chemical process used for converting oxides of nitrogen (NOX) with the aid of a catalyst into diatomic nitrogen (N2) and water (H2O).
In a vehicle equipped with an SCR device, a reductant, which may be an aqueous solution of urea, is typically injected into the engine's exhaust stream. Once in the exhaust stream the reductant is absorbed into the system's SCR catalyst, wherein the catalytic action of the SCR device ultimately converts NOx gasses into inert byproducts, i.e., nitrogen and water.
The exhaust stream is typically filtered with a particulate filter, in a diesel application a diesel particulate filter (DPF). The DPF, may be positioned upstream or downstream of the SCR device and captures engine soot and other suspended particulate matter before it can be discharged via the tailpipe. Overtime, soot loading accumulates in the porous media of the DPF. In-situ thermal regeneration of the DPF is therefore conducted periodically to burn off accumulated particulate matter.
A diagnostic for the SCR device is utilized to detect the overall NOx removal efficiency. Due to a high degree of confidence that is required in the levels of NOx gasses discharged by vehicles, certain government agencies require periodic monitoring of the NOx removal system efficiency to ensure the overall integrity of a diesel exhaust system, as well as to comply with government agency regulations.
The accuracy of the diagnostic for the SCR device is necessary to ensure correct reporting and maintenance of the diesel exhaust system in compliance with government agency requirements. It is also important to the consumer that the diagnostic produce only valid notifications indicating a failing emissions result.